Comet Halley Develops a Tail
17 December 1985
Comet Halley was photographed with the ESO 1 metre Schmidt telescope at La Silla on 1985 December 9. The exposure was 10 min on a blue-sensitive emulsion. The telescope was guided on the moving comet. The stars in the field are therefore seen as short trails.
Although Comet Halley is somewhat brighter (magnitude 4 on 12 December) than originally predicted, it has been slow in developing a tail. This negative picture, which has been somewhat enhanced for clarity, shows two tails pointed towards East (away from the Sun). The thin, very straight tail (the northernmost) is a typical ion-tail, consisting of charged particles, which are pushed away from the comet by the solar wind (charged particles travelling away from the Sun at high speeds). The other ion tail, which is slightly bent and broader, can be followed to a distance of about 2.5 degrees (more than 5 million kilometres) from the comet's head. The bend (“kink") is due to a change in the solar wind direction. Both tails are enveloped in a very faint cloud of dust particles, also released from the comet.
When the picture was taken, Comet Halley was about 200 million kilometres from the Sun and 110 million kilometres from the Earth. It is moving south in the sky and is becoming more and more difficult to view from Europe. In early February, it disappears from view, when it passes behind the Sun. It is expected that it can be seen again around 15 February.
Many of the ESO telescopes will be used for observations of Comet HALLEY during early 1986.
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